DESCRIPTION: The human hereditary disease xeroderma pigmentosum (XP) is characterized by a profound predisposition to cancer. This extreme predisposition is noted primarily in the skin, because this organ is most frequently exposed to the prevalent carcinogen, sunlight. Some individuals with mutations in selected XP genes manifest the clinical phenotypes of at least two other hereditary diseases. One of these is called trichothiodystrophy (TTD) and is characterized by brittle hair, reduced intelligence, and impaired growth. In recent years it has been shown that cells from a number of TTD individuals are defective in DNA repair and carry mutations in the XPD or XPB genes. TTD patients are not prone to skin cancer and they do not show the typical clinical features of XP. It is remarkable that two clinically distinct human hereditary diseases can share defects in the human XPB and XPD genes. It has recently been shown that the proteins encoded by these two genes are bifunctional. They are known to be indispensable for the process of nucleotide excision repair (NER). Additionally they are subunits of the RNA polymerase II transcription factor IIH (TFIIH) and hence are also indispensable for transcription initiation. These observations have led to the notion that the complex clinical features of XP and TTD, and the presence or absence of a cancer predisposition in these syndromes may reflect the consequences of subtle transcription defects. The specific aim of this proposal is to generate mouse strains that carry subtle mutations in the XPD gene which mimic those established in known TTD individuals, and which are compatible with embryogenesis and postnatal development. The mice will be generated using gene replacement strategies involving homologous recombination in embryonic stem (ES) cells. Normal, heterozygous mutant and homozygous mutant mice will be investigated for evidence of the TTD phenotype, for defective NER and for cancer predisposition. Detailed studies on RNA polymerase II transcription in vitro will be carried out to determine whether the molecular pathology correlates with reduced expression of a particular subset of genes.